The invention relates generally to solar control members for coating windows and the like, and relates more specifically to applied window film members which provide solar rejection and low visible reflection.
Various films have been applied to windows to reduce glare and to obtain solar screening for an interior of a structure, such as a home, building or car. For example, a plastic film may be dyed to provide desired optical properties or may be coated with a number of layers to acquire the optical properties. A film that provides solar screening is one that has a low transmission in both the visible range (400 to 700 nm) and the near infrared range (700 to 2100 nm). To reduce glare, the transmission of visible light (TVIS) must be controlled.
Primarily through absorption, dyed films can be fabricated to provide a wide range of TVIS values. However, dyed films generally do not block near infrared solar energy and, consequently, are not completely effective as solar control films. Another shortcoming of dyed films is that they often fade with solar exposure. When the films are colored with multiple dyes, the dyes often fade at different rates, causing unwanted color changes over the life of the film.
Other known window films are fabricated using vacuum-deposited grey metals, such as stainless steel, inconel, monel, chrome or nichrome alloys. The deposited grey metal films offer about the same degrees of transmission in the visible and near infrared portions of the solar spectrum. As a result, the grey metal films are an improvement over dyed films with regard to solar control. The grey metal films are relatively stable when exposed to light, oxygen and moisture, and in those cases in which the transmission of the coatings increases due to oxidation, color changes are generally not detectable. After application to clear float glass, grey metals block light transmission by approximately equal amounts of solar reflection and solar absorption.
Vacuum-deposited layers such as silver, aluminum and copper control solar radiation primarily by reflection. Because of the high reflection in the visible spectrum (i.e., high RVIS), films having these vacuum-deposited layers are useful in only a limited number of applications. A modest degree of selectivity of transmission in the visible spectrum over transmission in the near infrared spectrum is afforded by certain reflective materials, such as copper and silver.
Traditionally, the best glare reducing coatings have been sputtered grey metals, such as stainless steel, chrome and nickel. The graph of FIG. 1 is a transmission spectrum 10 for a sputtered nichrome coating that is designed to transmit approximately 50% of the light at the center of the visible light spectrum (i.e., TVIS=50%). The nichrome is affixed to a 3.2 mm-thick plate of float glass. As can be seen, the transmission of energy is controlled in both the visible and near infrared portions of the solar spectrum. A slight degree of wavelength selectivity is observed due to the iron oxide in the glass.
In the graph of FIG. 2, the visible reflectivities of single and double layer nichrome films of various thicknesses are shown as a function of the corresponding visible light transmissions. (Here, double nichrome films refer to a construct in which two optically isolated sputtered coatings are employed, with the films being separated from each other by a relatively thick (22 micrometers) layer, such as a laminating adhesive.) While not shown in FIG. 2, the nichrome layer thicknesses decrease from left to right. As can be seen, the RVIS value decreases and the TVIS value increases as the nichrome layers become thinner. The comparison between the single and double layer nichrome films evidences that the double layer of nichrome has a substantially reduced RVIS value for the same TVIS value. For example, at a TVIS value of 20%, the single nichrome coating has an RVIS value of 24%, while the double nichrome coating has an RVIS value of 13%. As the nichrome layers become thinner, the RVIS values of the two films converge.
The percentages of solar rejection achieved by films with single and double layers of nichrome are compared in the graph of FIG. 3. Solar rejection is defined as:
solar rejection=solar reflection+(0.73xc3x97solar absorption). 
Within the art, solar rejection is often calculated using solar energy distributions as given in the ASTM E 891 method. The slightly better solar rejection noted for the low transmission single nichrome coatings relative to the twin nichrome equivalents is due to solar reflection differences.
A low visible light transmission and low visible light reflection film utilizing double layers of nichrome is disclosed in U.S. Pat. No. 5,513,040 to Yang. The patent discloses a solar control film having two or more transparent substrates, each bearing a thin, transparent and discontinuous film of metal having low RVIS and a degree of visible light blocking capacity. The substrates are arranged and laminated into a composite, such that the visible light blocking capacities of the metal films are effectively combined to provide a composite having low visible light transmittance, i.e., a low TVIS. The discontinuous films of nichrome are attached using an adhesive layer.
The possibility of using metal nitride films in window-energy applications was discussed by C. Ribbing and A. Roos in an article entitled, xe2x80x9cTransition Metal Nitride Films for Optical Applications,xe2x80x9d which was presented at SPIE""s International Symposium on Optical Science, Engineering and Instrumentation, San Diego, July/August 1997. Single layers of TiN, ZrN and HfN were specifically identified. The article discusses the use of the materials in low emissivity coatings to replace noble metals, such as silver and gold. It is noted that the low emissivity coatings will not reach as high a selectivity as the current noble metal-based multi-layers, but may find use in aggressive environments, because of their excellent stability.
What is needed is a solar control member for application to a window or the like in order to achieve a high selectivity of visible transmission to near infrared transmission, with a controlled visible reflection and with age stability. What is further needed is a repeatable method of fabricating such a solar control member.
A solar control member utilizes a combination of layers that include spaced apart titanium nitride layers in order to achieve a desired combination of optical characteristics, including characteristics relating to visible transmission (TVIS), near infrared transmission (TNIR) and visible reflection (RVIS). Adjacent titanium nitride layers are spaced apart by a distance that promotes optical decoupling with respect to constructive and destructive interference of visible light propagating between the two titanium nitride layers. In the preferred embodiment, each titanium nitride layer is formed on a separate substrate, such as a PET substrate, with first and second titanium nitride layers then being joined by a laminating adhesive having a thickness greater than the wavelengths associated with visible light (i.e., greater than 700 nm). It is recommended that the distance between the titanium nitride layers be at least 1000 nm, with 3000 nm being more preferred. In another embodiment, the first and second titanium nitride layers are formed on opposite sides of a substrate, such as PET, so that the substrate provides the recommended spacing between the two layers.
The thickness of each titanium nitride layer depends upon the desired optical properties. Preferably, the titanium nitride layers are sputter deposited in a manner that facilitates reproducibility, but allows an adaptation for varying the TVIS value within a range of 20 to 70% and more preferably within the range of 30 to 60%. The TVIS value is achieved while the RVIS value remains below 20%. Moreover, the ratio of transmission at the wavelength of 550 nm (T550) to transmission at the wavelength of 1500 nm (T1500) is at least 1.25. That is, the selectivity as defined by T550/T1500 exceeds 1.25.
In most applications, two sputtered titanium nitride layers are sufficient. Individual TVIS values for the films should be within the range of 45 to 70%, so that the dual film laminate structure has an RVIS close to 10%. However, to obtain a composite visible transmission of less than 40% while maintaining the individual film visible transmissions within the range of 45 to 70%, a third sputtered titanium nitride layer may be necessary. More than three sputtered layers may be required to obtain a composite visible transmission of 20%.
Greater wavelength selectivity is obtained if the titanium nitride layers are combined with transparent oxides (e.g., oxides of tin, indium, zinc, titanium, niobium, bismuth, zirconium, or hafnium) or nitrides (e.g., silicon or aluminum nitride) having a refractive index at least as great as that of the substrate material (the refractive index of PET is 1.7). The transparent oxides or nitrides can be placed on one or both sides of the titanium nitride. The thickness would range between 10 and 60 nm, depending upon color, reflectivity, or cost requirements. A transparent nitride, such as silicon nitride, is preferred over an oxide, since during the deposition process xe2x80x9ccrosstalkxe2x80x9d between the titanium and silicon processes is less likely to introduce excessive oxygen into the titanium nitride layer.
In all practical vacuum web coaters, some incorporation of oxygen will occur, so that in practice what is deposited is actually titanium oxynitride. However, if excessive oxygen is incorporated into the titanium nitride coating, the wavelength selectivity and electrical conductivity will be lost. It is believed that the oxygen-to-nitrogen partial pressure ratio during the sputtering process should be less than 0.5. To ensure that wavelength selectivity and electrical conductivity are achieved, the stoichiometry of each titanium nitride layer must be controlled. Two of the concerns with regard to adversely affecting the titanium nitride performance are (1) ensuring that each layer does not become too metallic and (2) ensuring that excessive oxygen is not incorporated into the layers. In either case, the wavelength selectivity will be lost if the sputtering process is not properly performed. A titanium nitride layer will become too metallic if it is nitrogen depleted, as will occur if nitrogen flow during the process is inadequate. The exact nitrogen flow to achieve a suitable titanium nitride layer varies from coater to coater. However, the most preferred flow generally corresponds to minima in sheet resistance and absorption at 1500 nm. As the nitrogen flow is being adjusted, if a T1500 value is to be maintained with an increase in nitrogen flow, the linespeed of the deposition process should be reduced. This is largely due to a decrease in the deposition rate of the titanium nitride.
Regarding excessive oxygen, the extra oxygen typically comes from background water and oxygen contaminants present in the sputtering system. The problem is enhanced if other oxygen-requiring processes (e.g., plasma pretreatments or reactive sputtering) are conducted in the vacuum chamber while the titanium nitride deposition process is being conducted. To reduce the likelihood that contamination will occur, the following steps may be taken (1) sputter as fast as possible using high powers and the minimum acceptable nitrogen flow, since excessive nitrogen xe2x80x9cpoisonsxe2x80x9d the titanium target and reduces the deposition rate; (2) minimize background contamination by controlling xe2x80x9ccrosstalkxe2x80x9d between neighboring processes, by minimizing the water content in the substrate (for example by preheating or separate outgassing steps), by eliminating any water leaks in the vacuum chamber, and by adequately pumping down the vacuum system prior to beginning the deposition; and (3) sputter through a mask, so that the outer perimeter of the titanium nitride plasma (which deposits on a mask, rather than the substrate) acts as an xe2x80x9coxygen getter.xe2x80x9d
Optionally, the surface that is exposed when the solar control member is attached to a window is protected by a hardcoat layer. Hardcoat layers are known to provide resistance against abrasion. Another optional layer is a low surface energy layer on the hardcoat. The low surface energy layer acts as an antisoiling layer for resisting smudges and the like and as a lubrication layer for improving the resistance to mechanical abrasion.